The invention relates to a backlash-free gearbox, for example, for a main spindle of a machine tool, preferably as a reduction gear for a work piece spindle of a gear hobbing or a gear grinding machine.
The machining of gears requires a very precise and complicated relative movement between tool and work piece which consist of several individual rotational and linear movements. In the continuous gear machining processes, besides the work piece, also the usually fast rotating tool has to be coordinated with respect to its angular position at every point in time with the rest of the relative movement proportions. While, in the past, complicated and highly precise mechanical gear trains were necessary to coordinate all of the various movements, today""s control technology allows to drive every movement via a separate NCxe2x80x94axis-mostly equipped with a direct measuring systemxe2x80x94with selectable coupling factors.
Because mainly the rotating angle precision of the workpiece during the machining, and thus the rotating angle precision of the workpiece spindle, is decisive for the quality of the finished toothing, drives are suitable for this only if they have, besides very smooth operation, a high rotational rigidity and are completely free of backlash. In the past, it has been attempted in a variety of ways to build drives considering these stringent requirements. For continuous hobbing machines, often low-backlash or backlash-free worm gears have been employed for this purpose, and in generating grinding machines often backlash-free spur gears or simple friction wheel gears. In machines, in which only small torques act on the workpiece spindle, also direct drives via backlash-free couplings can be found. Such a direct drive has many advantages: on the one hand, a high rotary stiffness free of backlash between motor and workpiece spindle is obtainable without much effort; on the other hand, the complexity of the design in comparison to the solution using gears is small.
If, however, larger gear wheels are machined on a machine, firstly, usually the required maximum speed of the workpiece is smaller and, secondly, the torque caused by the process forces are larger. For practical but mainly for technical reasons it is more advantageous in those cases to use a smaller motor coupled to a reduction gear for the drive of the workpiece spindle, than a direct drive with a correspondingly large motor. This is important from a control and technical point of view because, with a reduction gear, the torque acting on the workpiece spindle is reduced linear, the inertia of the workpiece, however, is reduced with the square of the gear reduction ratio on the motor shaft.
Worm gears are often not considered anymore because their gear reduction ratio is too large for the required ranges of speed of the workpiece spindle. Additionally, a really backlash-free worm gear produces a lot of friction which results in a low efficiency and generates undesirable heat at a sensitive location. Spur gears have the disadvantage, even if they have been produced with the highest care and precision, to generate disturbing tiny oscillations of the gear tooth meshing frequency on the workpiece spindle. While friction wheel gears have a very good transfer quality, they are usually limited in regard to torques transfer, because they often have only one single friction force transfer point. In addition, means for force compensation are necessary if the workpiece spindle bearings should not receive the axial or radial pressing force of the friction wheel.
It is the object of the present invention to disclose a backlash-free gear which has a smooth run, can transfer high torques and has a high rotary stiffness.
The present invention describes a workpiece drive which has the advantages of a friction wheel drive, namely the very smooth and shock-free rotational angle transfer, but does not have the disadvantages of a limited torque transfer and the necessity of a force compensation device and can also be built for a high rotary stiffness.
The drive according to the invention is, in terms of the design, a planetary gear in which the annular wheel is mounted rigidly to the housing and generates via a suitable deformation simultaneously the pressing force between the annular wheel and the planetary wheels on the one hand, and the planetary wheels and the sun wheel on the other hand.